feature-dev

feature-dev

Implement the smallest safe change that satisfies a PRD's acceptance criteria using a test-driven loop.

When to use

• The user says "implement this", "build this feature", "let's write the code".
• A workflow invokes polymath-engineering:feature-dev.
• A PRD or acceptance criteria exist and the next step is code.

Inputs

• PRD path (preferred) or a concise problem statement.
• Existing acceptance criteria (preferred). If absent, surface that gap and pause.
• Repository conventions (loaded from polymath-core:conventions if installed).

Procedure

1. Orient. Read the PRD and the surrounding code area. If unfamiliar, invoke the read-code skill first.
2. Pick the smallest acceptance criterion that is independently testable.
3. Write a failing test for that criterion. Use the project's test framework. If no test infrastructure exists, surface this and ask before scaffolding.
4. Run the test. Confirm it fails for the right reason (assertion failure, not import error).
5. Implement the minimum code to pass that test. No speculative generality.
6. Run all relevant tests. Confirm green.
7. Refactor only the new code. Inline obvious duplication; do not refactor surrounding code.
8. Repeat 2–7 for the next acceptance criterion.
9. Stop when acceptance criteria are met. Do not invent additional features.

Quality bar

• Each commit (or logical chunk) keeps tests green.
• No dead code, no half-finished alternatives, no comments narrating the change.
• No new dependencies without an explicit ADR (out of v0.1 scope; surface as an open question).
• Test names describe the behavior under test, not the implementation.

Output

• A code diff that satisfies the listed acceptance criteria.
• Tests (unit or integration) co-located per repo conventions.
• A short summary listing files changed and behaviors covered.

Anti-patterns to avoid

• Implementing behavior not in the PRD ("while I was in here…").
• Adding error handling for cases the PRD does not call out.
• Refactoring code outside the change set.
• Adding feature flags or compatibility shims when a direct change suffices.